Questions about imaging the LMC

[thegableguy]

I took this last week - full frame DSLR on 8" f/5. Don't remember but probably about 2hrs of 2-min subs at ISO 400.

I was hoping for more of the surrounding galaxy-ness if you know what I mean. It just looks like a star field in our own galaxy rather than the lovely wispy clouds a Google image search for the LMC produces.

Is it just a matter of exposure? That's my best guess - I know the Tarantula is incredibly bright, so perhaps it has to be blown out for the much fainter bar of the LMC to show. Or maybe I could do 2-3 different exposures and combine the resultant final images in Photoshop...? Or does it work okay stacking multiple exposures all at once in DSS? That's the easiest way but I suspect the least successful.

Tips appreciated!

[doppler]

Nice Tarantula. The LMC is quite large so you are only capturing a small section. 135mm to 200mm focal length is ideal.
I have a streetlight in that direction and a couple of trees so can only get a few short exposures, but here is a total exposure of maybe 5 mins with a piggy backed 135mm 1100d as an example of frame size and the other is with a 50mm full frame 35mm film camera, (the 1987 supernova is next to the Tarantula)

[pluto]

As Rick said, I think you need a wider field of view, and probably longer exposures (or higher ISO).
This was about 30 x 60 second exposures on an A7s at ISO1250. The scope was a William Optics Star71 so the focal length is about 350mm.

[thegableguy]

Thanks fellas.

I know that 1000mm is far too small a FOV to get the whole thing; it was more a question about exposures to get the fainter stars. However, from looking at your photos I think the fact is that the Tarantula is in a different part of the LMC to the main bar where the fainter stars are, which is why they're not showing up at all. For some reason I thought the Tarantula was more in the middle of it all, not quite as separate as it is.

Might try a crop sensor with a 200mm lens. It'll be fun to go really wide for a change.

[glend]

That's a nice shot for an un-modified DSLR. Keep im mind that the LMC is high is Ha and Oiii narrowband light. Your DSLR can pick up the Oiii (which is on the blue/green boundary), but very little of the Ha ( which is the deep red part of the spectrum). Typically an un-modified DSLR can only capture about 20% of the red spectrum due to the internal filters. A wide field Ha shot of rhat same area will show you much more, and many hidden structures. The whispy clouds you see in the LMC are usually Ha emission nebula structures. It will not help much to put an Ha filter on your DSLR as the internal filters will still block most of it. An astro modified DSLR will allow about 5x as much Ha light through and this will give you the whispy structures you seek.

[thegableguy]

I was thinking of modding my cheap Nikon but that would basically rule it out for any other kind of photography, yeah? And I'd have to use filters to get colour images, and basically quadruple the capture time to get the different channels? Do I have that right? Or am I supposed to just add Ha to the unmodded DSLR capture?

[Somnium]

A modded camera just removes the ir filter, , not the Bayer matrix so you will still capture colour images, though you may need to adjust the white balance to make the colours more natural

[glend]

Well actually most DSLRs have more than one filter. Yes there is an IR Cut filter that Aidan refers to below, but there is usually a blue cast filter (sometimes referred to as LP2) that skews the camera towards producing good daytime colour. The Bayer Matrix does stop alot of the red recognition, due to the preference in the matrix design given to blue and green pixel filter layers. If you look at the illustration here:

https://en.wikipedia.org/wiki/Bayer_filter

it shows you what I mean.

Doing an Astro filter mod on a stock DSLR will open it up to more Ha light but your still relying on a small number of Ha capable pixels to capture it.

A full mono conversion of a DSLR (a De-bayering of the sensor) will open it up to narrowband imaging, where every pixel can capture Ha with the appropriate filter on the front. However, it will be useless as a standard camera.

[thegableguy]

Ah okay. Yeah the full debayering conversion was what I was thinking of, didn't realise there was an option in between. The simple version of removing the IR filter might be the way to go; I think that if I stick with AP long enough to progress from DSLR imaging, I'll get a proper cooled CCD.

From what I can tell of reviews of the D810A and the 60DA there's not actually a huge difference achieved by removing the IR filter. There's a difference, sure, but it's not that significant. Is that a fair representation, or am I reading the wrong reviews? They seem written by photographers rather than dedicated APers so they're possibly unaware of how to pull the best results from stacking & processing.

[glend]

Well it depend on what you are trying to achieve. An IR Cut filter stops infrared which is below Ha and Sii on the spectrum. If you leave the camera open to IR then your imaging in IR, which can over power in my opinion.

[rustigsmed]

yes, as glen says there is a lot of Ha in the tarantula. astro modifying your camera will help heaps.

not quite a side by side comparo but you can see a difference from a couple of my old dslr shots

here is 1 hr of 2 min subs with astro modification 1200mm FL.
https://www.flickr.com/photos/80336656@N07/16049637565/in/photostream/lightbox/


and here is one with my old goto dob 44x 20 second subs non-astro modified
https://www.flickr.com/photos/80336656@N07/12163421945/

but really full NB really makes the emission neb's pop out from the bright background stars.

[raymo]

Chris , regarding your original questions, yes you can stack subs of
different duration successfully in DSS; I also routinely stack subs
of different ISOs.
As you can see from my attached pic, 2070 is near the bottom edge of
the LMC, but far enough in for your entire 1000mm f/l image to be
within the galaxy, hence no change of star colour or density in your image. Mine was taken at 50mm, so you can see that nothing longer than around 85mm wil make a pleasing image of the whole of the LMC.
raymo

[raymo]

Sorry to nit pick Russ, but you list the second pic as taken with a non modded camera, but the script accompanying the image says the camera was modded.
Judging by the fact that 2070 is showing only a little of the green that non
modded cameras usually produce, I'm guessing that the script is right.
raymo

[rustigsmed]

thanks Raymo, hmmm I suspect that it isn't modified but could be wrong. That version I revisited it when I had more advanced processing skills and it may have been that I guessed it being modified at the time of uploading.

here is an even older version here (pretty sure it was same data) and note its not mentioned as astro modified and wouldn't have been in 2012. much more black clipped but i'll see if I can definitely find out on that other pic


https://www.flickr.com/photos/80336656@N07/8260374701/in/album-72157634243605994/